Method of exhausting vacuum tubes



July 2, 1929- vH. E. METCALF 1.719.773

METHOD OF EXHAUSTING VIACUUM TUBES Filed Oct. 13, 1924 Seal.

l y i E i 6'Ig/VENTUR. '5W/hf M 'ik Patented .uly 2, 192i).

UNITED STATES i f inane PATENT orifice, i

HERBERT E. METGALF, OF SAN LEAITIDRO, CALIFORNIA, ASSIGNOR TO THE MAGNAVOX C0., OF OAKLAND, CALIFORNA, A CORPORATION OF ARTZON'A.

METHOD' 0F EXHAUSTING VACUUM TUBES.

Application led October 13, 1924.

The common p actice among those skilled in the art of exhausting vacuum tubes is to heatthe electrodes contained therein While under a high vacuum, such heating being of considerable assistance in exhausting the tube. This is customarily done by means of eddy currents applied from outside the tube or vessel. This heating of the electrodes is also used to vaporize various chemicals lino Wn as getters, inside the vessel. The heating of the internal eleetrodesis said to free said electrodes from residual gas and as the vessel is under the pull of the pump, said gases are thereby removed.

` My invention provides a means of heating said electrodes under avery low vacuum, provides greater heat and speedier exhaust as Well as vaporization of getters, all under low vacuum With large quantities of gas present. lt has the advantage of giving a vehicle of gas to carry out the freed gases, of heating any and all electrodes, Whether or not they happen to be shielded from eddy currents, With maximum speed.

In the accompanying drawing Figure l il lu'strates a tube such as is contemplated by my prior Patent No. 1,558,111, dated October 9.0, 1925, and Figure 2 is a graph showing the steps involved in carrying out my present invention.

Following is a description of my process: The vessel or tube Will besupposed to have three electrodes, such as the common three electrode vacuum tube. A tube such as is illustrated in the accompanying dravving may be prepared in accordance with my present invention and, in fact, all other vessels With two or more electrodes may be treated in the same Way. The vessel is heated, before exhausting, to the maximum temperature which the container can stand; in other Words, baked out. Tt is then attached toA the pumps, Which consist of a rotary preliminary pump and an ordinary rotary molecular oilfilled vacuum pump as a final.

The vessel is brought up to a vacuum whereby the filament may be burned in safety, and then the vacuum is at once dropped back to the preliminary. High frequency oscillations generated by any of the Well-known means are then passed between the grid or control electrode and the anode. The space betvveen the electrodes becomes ionized and the electrodes immediately be- Eelial No. 743,388.

come redhot by impact of the gas ions. The getter, which was attached to the elec`- trodes, then becomes vaporized and deposited on the Walls of the vessel.

As soon as the electrodes are sufficiently heated the current is turned ofi and the vessel is thrown onto the final pump, which pumps to its limit, and before the electrodes and the deposited film have had time to re-absorb any panying drawing, it will be seen that the tube is first evacuated to a high degree and that the filament is then burned in this vacuum. The reason for this high vacuum during the burning of the filament is because the lilan'ient Will become damaged and possibly burn out unless a high degree of vacuum is maintained at this time. This operation thoroughly degasiiies the filament. lifter this step the vacuum in the tube is allowed to fall to `a low degree 'anging from one to three millimeters in pressure. The grid and plate are then energized by circulating a high voltage or high frequency current between the tvvo. The loiv vacuum duringl this operation facilitates the conduction of current through the gases in the tube and materially assists in heating the grid and plate by this method. During this period of my process, the grid and plate obtain a. very high temperature Which drives out the gases therefrom and also volatilZ-es the getters applied thereto. This operation is carried on for approximately twenty seconds and then the tube is again evacuated to the maximum degree possible by the pump and is then sealed oif. It might be stated that after the tube is sealed off, the cooling of the grid and plate creates a further degree of vacuum and, as a result, a very satisfactory tube is produced.

I have found that if the preliminary burning and degasiiication of the filament is carried out as above, the damage to the filament is prevented when high frequency or high voltage current is passed between the other electrodes. This damage takes the form of pitting of the filament, due to the instant ionization of the gas at the surface of the filament when the same has not been previously degasiiied.

The entire process of pumping, from the time the vessel is attached to the pump until it is tipped off, is by my method less than two minutes, and the bulb is never allowed to cool down. No ovens or external heating devices are used.

It is quite natural that some leakage of the high frequency current should take place on the leads inside the tube and close to the vpinch of the stern. This leakage heats the leads to the point that the glass of the stem is apt to be heated so hot'that it cracks, or becomes conductive and shorts as it becomes hot, With consequent loss of the vessel. My invention provides coating the leads and the inner part of the pinch With a heat resisting non-conducting protection. This protection which consists of a cement, suehi as alundum cem-ent, or other refratory material, absolutely prevents arcing or shorting between the leads, and confines the heating effect Where it belongs, on the electrodes.

My invention heats all electrodes evenly. Then eddy currents are used to heat a cylinder With a grid inside, the cylinder takes all the current and becomes hot, While the grid is shielded and does not become hot. ,My method heats both of them to a red or even a White heat if desired, With consequent greater freedom from residual` gas. Furthermore, it is the inside electrode which is closest to the filament and which, therefore, gets the hottest under normal operation of the tube, and is therefore apt to contaminate the tube with gas after it is made. My method heats the grid as hot as the other electrodes, much hotter than it Will ever get under normal conditions again.

Having thus described my invention, what I claim and desire to secure by Letters Patent is- 1. The method of exhausting electrode vacuum tubes or vessels, which consists of heating the electrodes and vaporizing gettersV under low vacuum only, thereafter pumping to the limit of a molecular pump, and sealing oft' before the heat of processing has been lost.

2. The method of exhausting electrode vacuum tubes or vessels, Which consists of heating the electrodes to free residual gas and vaporize getters, thereafter stopping the heating and pumping as high as possible to remove such gas and gctters and to create a high vacuum, and sealing While the vessel is still hot.

3.. The method of exhausting electrode vacuum tubes or vessels, Which consists of electrically heating all of the electrodes to a high heat by passing a current through certain of such electrodes before forminga high vacuum in the tube, thereafter stopping the current and setting up a high vacuum to exhaust the tube, and finally sealing the tube While the electrodes are hot.

4. The method of exhausting a radio vacuum tube having three electrodes therein, comprising a grid, a filament, and` a plate, which consists of electrically heating all of the electrodes to a highheat by passing a current through the grid and plate before, foi-n1- ing a high vacuum in the tube, the said grid being heated to a temperature higher than it normally attains in use, thereafter stopping the current and setting up a high vacuum to exhaust the tube, and inally sealing the tube while the electrodes are hot.

HERBERT E. METCALF. 

